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In-Reply-To: <00c901d74543$57fa3620$07eea260$@samsung.com>
From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Mon, 10 May 2021 09:19:43 -0400
Message-ID: <CA+FuTSepShKoXUJo7ELMMJ4La11J6CsZggJWsQ5MB2_uhAi+OQ@mail.gmail.com>
Subject: Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6
 to 4
To:     Dongseok Yi <dseok.yi@samsung.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Yunsheng Lin <linyunsheng@huawei.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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> > That generates TCP packets with different MSS within the same stream.
> >
> > My suggestion remains to just not change MSS at all. But this has to
> > be a new flag to avoid changing established behavior.
>
> I don't understand why the mss size should be kept in GSO step. Will
> there be any issue with different mss?

This issue has come up before and that has been the feedback from
TCP experts at one point.

> In general, upgrading mss make sense when 6 to 4. The new flag would be
> set by user to not change mss. What happened if user does not set the
> flag? I still think we should fix the issue with a general approach. Or
> can we remove the skb_increase_gso_size line?

Admins that insert such BPF packets should be aware of these issues.
And likely be using clamping. This is a known issue.

We arrived that the flag approach in bpf_skb_net_shrink. Extending
that  to bpf_skb_change_proto would be consistent.